Process of and apparatus for separating ternary gas mixtures



Apnl 3, 1951 G. ESIMPSON 2,547,377

PROCESS OF AND APPARATUS FOR SEPARATING TERNARY GAS MIXTURES Filed Nov.2, 1948 2 SheetsSheet l INVENTOR ATTRNEY GRAHAMESIMPSON- A rll 3, 1951 Ye. E. SIMPSON 5 9 PROCESS OF AND APPARATUS FOR SEPARATING TERNARY GASMIXTURES Filed Nov. 2, 1948 2 Sheets-Sheet a OXYGEN Z 20 &q

NITROGEN INVENTOR GRAHAM E.S|.MPSON j 2 BY I. a

, Q J, I x I I TTORNEY Patented Apr. 3, 1951 PROCESS OF AND APPARATUSFOR SEPA- RATING TERNARYGAS MIXTURES Graham E. Simpson, White Plains, N.Y., assignor to The Linde Air Products Company, a corporation of OhioApplication November 2, 1948, Serial No. 57,846

11 Claims.

This invention relates to a process of and apparatus for separatingternary gas mixtures of relatively low boiling point into theirconstituents by low temperature rectification, and more particularly tothe low temperature separation of air to provide oxygen, nitrogen, andargon prodnets, with the simultaneous elimination of certain undesirablehigher boiling point impurities which occur in the original gas mixtureor air in very small proportions.

As set forth in United States Patent No. 2,287,158 of E. F. Yendall, theresidual carbon dioxide and hydrocarbon impurities in air may beremovedprior to low temperature rectification of air by scrubbing the air afterit has been cooled to about its condensing temperature with a fractionof liquid air to entrain the impurities in the liquid air. Theimpurity-free air may be rectified, but the impurities must be removedfrom the liquid fraction. According to the aforementioned pat-ent, theimpurity removal is effected by evaporating a major part of theimpurity-containing liquid by heat exchange with a gaseous oxygenproduct to make liquid oxygen. The concentrated residue of theevaporation is withdrawn and discarded. Other methods of efiecting aheat exchange to evaporate and concentrate the impurity-charged scrubberliquid have been proposed, but in general such methods involve more orless loss of eiiiciency.

The recovery of an argon product when separating air by low temperaturerectification has been accomplished by withdrawing vapor from a part ofthe final air rectification where the vapor has a substantial argoncontent, and then subjecting such withdrawn vapor to an auxiliaryrectification. To provide reflux for such auxiliary rectification,various expedients have been proposed which generally incurred somelosses resulting in increased power requirements.

It is a principal object of the present invention to provide an improvedprocess of and apparatus for separating a ternary gas mixture such'asair with the simultaneous elimination of impurities having higherboiling points than those of the components separated. Other objects ofthis invention are to provide a process of and apparatus for separatingair to recover oxygen and argon with the simultaneous elimination ofimpurities collected in a liquid fraction of the air to be separated, inwhich the refrigeration produced by evaporation of theimpurity-containing air may be usefully employed for supplying a refluxliquidfor washing higher boiling components from an argon-rich vapor;and for integrating with the main rectification of air an auxiliaryrectification of argon-containing vapors and a preliminary separationfrom air to be rectified of higher boiling impurities in an economicalmanner, and which provides a system that is substantiallyself-regulating, easy to operate, and relatively simple to construct. Afurther object of the invention is to provide a method of and apparatusfor separation of air by low temperature rectification including theelimination of impurities from air by concentrating them in a liquefiedfraction of air in which a major part of the impurities which areentrained in a solid state in such liquefied fraction may be removed byfiltration and the remainder of the impurities subsequently removed bydistillation so that the amount of impurity-containing residue to bediscarded is reduced to an immaterial amount.

These and other objects and advantages of this invention will becomeapparent from the following description and the accompanyng drawing inwhich:

Fig. 1 is a diagrammatic elevational view of an v exemplary apparatusillustrating the principles of the invention; and i Fig. 2 is a similarview of an alternative system according to the invention for producinggaseous oxygen.

The present invention will be described in connection with the apparatusof Fig. 1 which is particularly suitable for the separation of air toobtain a liquid oxygen product free of higher boiling impurities and aproduct having a high content of argon. However, modifications Withinthe scope of the invention may be made as exemplified in Fig. 2 if it isdesired to recover the oxygen product in gaseous instead of liquid stateand/or simultaneously obtain nitrogen in a high state of purity. Also itis contemplated that the argon-enriched vapor may be subjected to anadditional auxiliary rectification if it is desired to obtain argonproduct of very high purity.

Referring now to the drawing and particularly to Fig. 1; the apparatusfor preparing a gas mixture or air for rectification may be any ofseveral known forms, that schematically illustrated being particularlysuitable when the oxygen product is to be taken away in liquid form. Amulti-stage compressor I t compresses air to a relatively high pressureand such compressed air after being cooled in a water-cooledafter-cooler H is passed through a conduit I2 to the inlet end of acompressed air passage l3 extending through a countercurrent heatexchanger M. In the heat exchanger I4 the compressed air is cooledsufiiciently to eliminate the moisture. Such moisture-removing heatexchangers are customarily provided in duplicate so that accumulatedmoisture can be removed from one of the heat exchangers while the otheris in service. A trap !5 with a drain valve 55 may [be mounted at thelower end of the passage 33.

From the trap the compressed air flows through a conduit ll having abranch is conneoted to the inlet end of a compressed air passage l9;extending through a liquefying heat exchanger 28. The cold liquefied airformed in the heat exchange passage i9 is conducted through a conduit 2!to a scrubber-separator chamber 22. The scrubber-separator chamber maybe similar to that disclosed in the aforesaid United States Patent No.2,287,158 and, as shown, comprises a chamber 22 within which are 'so'meperforated plates 23 or other means for efiecting intimate mixing of gasand liquid. The-conduit 2! enters below the plates 23 and there isprovided an overfiow cup 25 with its upper opening immediately above theupper plate The scrubbei separator 22 may also be provided with a drain25, the valve of which generally remains closed during operation.

A branch 26 of the conduit ll conducts a substantial proportion of thecompressed air to the inlet of an expansion engine 2?. This engineexpands the air passed therethrough with production of external work sothat the expanded air has a temperature substantially correspond ing toits condensing temperature at the pressure existing in thescrubber-separator, which is pref erably about the same as the pressureof the first stage of rectification. This is of the order of 70 p. s. 1.gauge. The expanded air is led by a conduit'28 from the expansion engineto the lower part of the scrubber-separator 2'2. Interposed inthe-conduit 25 is an expansion valve 2| through which the liquefiedportion of the compressed air is throttle expanded to the pressure ofthe scrubher-separator. The scrubber-separator 22 is preferablyconstructed with sufiicient space above the gas and liquid contact meansso that no drops of liquid are entrained "in the vapor at the top of thescrubber-separaton The vapor, free of higher boiling point impurities,passes through a conduit 29 from the toner the scrubbenseparator to thehigh-pr ssure stage'or lower column 351 or a two-stage air rectifyingapparatus.

This air rectifying apparatus, except as explained hereafter, isgenerally of customary construction. The lower column or high-pressurestage chamber 36 is closed at its upper end by a tube sheet 3! to whichare connected the tubes of a condenser 32. The chamber 30 may containcustomary columntrays 33 such as perforated plates which eliect intimatecontact with vapors rising in the column and reflux liquid flowing downthe column. 'Under the tube sheet 3i there is an annular shelf 34 forcollecting liquid condensed by the outer tubes of the condenser 32. Suchliquid that collects on the shell 84 is substantially pure nitrogen.

The upper column or low-pressure rectifying chamber 35 extends above thetube sheet 35 and is formed at the lower end to provide a chamber 36about the condenser 32 for effecting the boiling or liquid oxygentherein to produce vapors for the rectifying action in the upper column35. The oxygen boils at the low pressure of the upper column at atemperature which is lower than the condensing temperature or" nitrogenin the condenser 32 under the pressure of the lower through thecondenser column. The nitrogen collected on the shelf 34 is transferredthrough a conduit 35 provided with an expansion valve 34" to the upperend of the upper column This supplies reflux for the upper part of theupper column which is provided with the customary trays .31. Liquid thatcollects at the bottom of the lower column chamber 36 and which isricher in oxygen than air, is transferred by a conduit 38 controlled byan expansion valve 35' to an intermediate point of the upper column 35.

The nitrogen product of rectification leaves the upper column through aconduit 3% connected to the colder end of the heat exchanger 28 fromwhich a conduit id conducts the eilluent nitrogen to the cold end ofheat exchanger E4. The nitrogen escapes from the heat exchanger itthrough an outlet ll at substantially atmospheric temperature.

-When the upper column is in regular operation and the liquid oxygenwithdrawal at a valved outlet 52 from the chamber 36 is at a ratesuchthat high-purity oxygen is produced, it is found thata substantialratio of the argon oi the original air is contained in the vapors at anintermediate point of the upper column. At-such intermediate point avapor outlet conduit 43 isconnected to conduct the argon-containingvapor into the lower part of an auxiliary rectifying column 44. Thiscolumn may also contain gas and liquid contact means such as trays Q5.The argon-containing vapor passes upwardly through .the column M- inwhich it is washed with a'refluxliquid produced by a condenser 46 at thetop of the column M. The vapors on passing through thetubes of condenser46 are subjected to partial liquefaction, the liquid produced being richin argon and forming a suitable reflux for the auxiliary column 33. Theargon-rich vapor which is not condensed passes through the condenser 5-6and is withdrawn through a conduit'fil provided with a'control valve it.The liquid which eventually reaches the bottom of the columnid issubstantiallystripped of its argoncontent and is mainly oxygen, and thisliquid is preferably drained by a conduit ig into the upper column'35 ata point below the Vapor-conduit The liquid that overflows :into the cup:24 is withdrawn through a conduit d9 that conducts it to a filterPreferably there is provided an expansion valve ts interposed in theconduit 49 so that the scrubber liquid may be expanded to a lowerpressure before it is-filtered and thereby its temperature-will bereduced. The filter 5B is diagrammatically illustrated and usually isone of duplicate set arranged so that one filter may becleaned'while'another is in service. The filter is provided with a drainvalve 5i for use when cleaning the filter. The filtered scrubber liquidis passed by a conduit :52 from the filterfiil to a chamber '53surrounding the condenser lfi atthe top of the auxiliary column. Thisscrubber liquid, due to the iact that it contains a 'large proportion ofnitrogen, boils at a lower temperature than thecondensing temperature ofoxygen and therefore heat exchange readilyoccurs-to eiiectpartialcondensation of the vapors passing The vaporized portion of thescrubberliquid passes throughaiconduit 5 5 into the upper column T35soithat these vapors which are now free of higher boilingximpurities,may also be rectified. The.conduit '54 may also be provided vwitharegulatingvalve: 5.

Since a large partiof the higher boiling ;impurities will be removed bythe filter 50 and only "a small amount will pass into the chamber 53,

most of the scrubber liquid that enters the chamber 53 may be vaporizedand passed into the upper column 35, and there will be only a very slowincrease in the concentration of higher boiling point impurities in theliquid being evaporated in chamber 53. In order that such concentratemay be removed, preferably at infrequent intervals, a drain valve 55 isprovided, connected to a lower part of the chamber 53.

It is believed that the operation of the apparatus is clear from theabove description. It will be seen that the withdrawal of a vaporcontaining the constituent of intermediate volatility (for example,argon), washing down a major part i of the higher'boiling component(oxygen) from the withdrawn vapor and returning such liquid to the mainrectification, makes it possible to obtain the higher boiling componentfrom the chamber 36 in a high state of purity, the lower boilingcomponent from the top of the main rectification in a high state ofpurity, and to recover a concentrate of the intermediate boilingcomponent which, if desired, can be further purified or subjected toanother rectification to eliminate residual low boiling component.

The undesired or dangerous impurities are effectively eliminated byconcentrating them in a liquid from which they can be removed beforethey enter the rectification zones. In air separation the impurities areconcentrated in a liquid of high nitrogen content. By filtering the mainbulk of impurities from the scrubber liquid, a concentration of the verysmall remainder that may pass the filter in dissolved state may beeffected by evaporation of practically all the filtered scrubber liquidand draining off only a very small residue without increasing theimpurity concentration above dangerous limits. Effecting thevaporization of the scrubber liquid by heat exchange with vapor at thetop of the auxiliar column provides eflicient recovery of therefrigeration in the scrubber liquid.

When oxygen is withdrawn at 42 in the liquid state to make liquid oxygenproduct as in Fig. 1,

the proportion of the air liquefied is correspondingly high so that aneffective quantity of scrubber liquid is produced for both elficientscrubbing action and for refrigeration of the auxiliary column. If it isdesired to make gaseous oxygen, the make oxygen could be withdrawn fromthe chamber 36 in either the liquid or gaseous states, as illustrated inFig. 2. When the oxygen is withdrawn as liquid, it is heated countercurrently in the liquefying heat exchanger 20 by providing therethrough anoxygen passage 60 wherein the oxygen is vaporized to produce gaseousoxygen and the required amount of liquid air in passage l9. If theoxygen is Withdrawn from chamber 36 in gaseous state, the amount ofliquid air entering the scrubber at 2! will be much smaller and it willbe necessary to augment the scrubber liquid for efiicient refrigerationof the condenser 46 at the top of the auxiliary column 4.4. This canconveniently be accomplished by providing a valved by-pass connection 56between the bottom of the chamber 30 and the conduit 52.

In the embodiment of Fig. 2, the features similar to those alreadydescribed in connection with Fig. 1 are designated by the same referencenumbers. As previously mentioned in connection with Fig. 2, the heatexchanger 20 is additionally provided with a passage 60 for outgoingoxygen. Similarly the heat exchanger [4 is provided with an outgoingoxygen passage 6| which has an outlet 62 for the warm gaseous oxygenproduct. A conduit 63 connects the warm end of passage 60 with the coldend of passage 6!. The inlet 64 to the passage 60 is provided with aconnection 65 to the vapor space of the chamber 36 and a connection 66to the liquid space of chamber 36. These connections are controlled byvalves 65 and 66 respectively.

With valve 65' closed, and valve 66' adjusted, the liquid oxygen makewithdrawn from chamber 36 through connection 66 is vaporized in heatexchanger passage 60 to liquefy air in the heat exchanger passage [9.Obviously, due to the refrigeration thus recovered by such liquid oxygen7 evaporation, the refrigeration produced by compression and expansionmay be less than in the case of Fig.1. With valve 56 closed. and valve65 adjusted, gaseous oxygen is drawn from the chamber 36 and warmed inthe passages 60 and BI, and in such case the latent heat of vaporizationof the make oxygen is supplied by the condenser 32 instead of by the airin heat exchanger passage [9 so that the scrubber liquid is augmented byliquidrfrom the base of the chamber 30.

Obviously, if desired, the refrigeration contained in the argon productmay be recovered by passing it through special heat exchange passageswhich may be provided therefor through the heat exchangers 20 and M.

It will be seen that the systems described are efiicient in thatrefrigeration contained in the scrubber liquid is returned. Theauxiliary column is substantially self -regulating and the argonproduction is very simply controlled by regulating the withdrawal atvalve 47'. There is practically no interference between the adjustmentsfor controlling the oxygen purity and recovery and those for the argonproduction and the removal of argon makes it easier to obtain highpurity oxygen with good yield.

It is to be understood that changes in carrying out the above-describedprocess and changes in the apparatus illustrated may be made withoutdeparting from'the principles of the invention.

What is claimed is:

1. Process for separating air by low temperature rectification to obtainoxygen, argon, and nitrogen products which comprises compressing,cooling, and drying air to eliminate substantially all the moisturetherefrom; further cooling and partially liquefying such air; scrubbingthe cooled gaseous portion of the air with the liquefied portion toprovide an impurity-free gaseous fraction and an impurity-containingliquid fraction; rectifying said impurity-free gaseous fraction to forman oxygen product of desired purity and a cold gaseous nitrogen product;withdrawing from a zone of the main rectification where the argoncontent is high, a vapor comprising mainly oxygen and argon; subjectingsuch vapor to an auxiliary rectification by washing with a reflux liquidformed by partial liquefaction of vapor from said auxiliaryrectification; producing said reflux by a heat exchange between vapor ofsaid auxiliary rectification and said impurity-containing liquidfraction to substantially vaporize said fraction and form animpurity-containing concentrate; removing said concentrate; passing theimpurityfree vapors from said vaporization to the main rectification;withdrawing an argon concentrate from said auxiliary rectification; andutilizing at least the nitrogen product for countercurrently coolingincoming air.

22- A process In se a atin a .aqcqr in c aim 1 whi 9 d s t 261 92 5199sai -m in rect ficat on e used refl .l uidimm .sa da li y rect ficat on-A pro es tor eparati air acc rd n t claim l whichincludesthe step ofsubjecting the perature and a minor .part .is liquefied and intimatelymixed with the major part; separating the major andminor parts .so thatthe impurities are containedsubs tantiallyallin the minor liquid I part;rectifying the impurity-free major .part'to form a product of the higherboilingcomponent of desired purity, a product comprising mainly thelowest boiling point component, anda vapor at an intermediate zone ofthe rectification including the intermediate boiling point component;withdrawing said /apor from the intermediate-zone and subjecting it toan auxiliaryrec- 'tification by washing'with areflux liquid formed bycondensation of part of the vapor ofsaidauxiliary rectification,withdrawing anintermediate coiling point component productfromsaidpartial condensation; efiecting substantialilaporization of saidminor liquid parthy heat exchange with said vapor of the auxiliaryrectification to form said reflux liquid-andaccumulate a remainder.containing impurities; withdrawing such remainder to eliminate theimpurities;-and passing the vapor ofsaid vaporization to themainrectification.

A-process for separating a ternary'gas mixture-according to claim 4which includes the step or" subjecting the minor liquid part tofiltration for remov alof solidified impurities prior to saidvaporization. H

6. A process ior separating a ternary gas mixture according to claim.dwhich includesthe step of returning to the mainre'ctific'ation .aliquid remainder of thelrefiux liquid employed in the auxiliaryrectification.

'7. A processiorthe separation of a gasmixture of three.mainconiponentsincluding also .a

.minoramount. of higher boiling no it impurities which comprisesproviding such a mixture in conditions or" pressure and temperature inwhich a major part ofthe mixture is ,at condensation temperature and a in'iinor part is liquefiedand intimately mixed with the major part;separating the majorandminor parts so that the impurities are containedsubstantially all in the minor liquid p r su e n th mnu ity-f s majorpart to a main rectification in twostages, the products of a first stagebeing passed to a second stage and the second stage formingla .cr d ctth h- 'h nb il g compo en .o fdc- .oQI i Sation'of part of the luap'oror said" to filtration to :remove at least pa jderffto "eliminate .th'mpuu res,

iary rectnicatio'nl' "or such"mixture in conditions jofpressutemperature in which a 'majorrpa rt purity drain ,at a low part of thehole,

.cludes means for draining refiuxli ..bottom of the auxiliary,columnll'to .sa1 cclumnl clu des a filter interpose d in said rneans toingthe liquid fraction to theb'oiler';

sired purity, a product comprising mainly the lowest,ho'il'ingjpoin'tlcomponentfjand a" apior' at tion hy washing with aboilingpointcempoiient H tial condensation i subjecting" said purities;then efie'cting'fsuhsgtantial v per;

or said minor liquid part bylh'eatiex 's'aid ilapor of theauxiliaryflfec'tific said reflux liquid .and accumulate a re aincontaining impurities} withdr' 'ing s vaporof saiuvam p 8. A,process forseparating a ter tui e according to claim .7 which inc or returning tothe mai rectification remainder of .therefiux liquidjemployed 91Apparatus for ,the separation of a ternary gas mixture containingimpurities whiohfapr ratu'sfcornprises means for providing f asupply othe mixture is atcondensationtemperature andiafminor part is'liquefiedand mixedi'with' the j means for separating such'supplyii toj 'an'fim-.purity-free vapor fraction ,and a liquidfraction containing theimpurities; a, mainiectfy n .urnn'; means for passing theyap'pr fractionto the rectifying 'columnjior recti cation therein; an auxiliaryrectifyin column having ref ujx assing vapor from an 1 ph a vord ns.tecia 11'. Apparatus according to claimjl which in- GRAHAM E, sturgeon.

.n FER NQES C ED The following references are .of record in the iile of.this patent:

UNITED STATES PATENTS

1. PROCESS FOR SEPARATING AIR BY LOW TEMPERATURE RECTIFICATION TO OBTAINOXYGEN, ARGON, AND NITROGEN PRODUCTS WHICH COMPRISES COMPRESSING,COOLING, AND DRYING AIR TO ELIMINATE SUBSTANTIALLY ALL THE MOISTURETHEREFROM; FURTHER COOLING AND PARTIALLY LIQUEFYING SUCH AIR; SCRUBBINGTHE COOLED GASEOUS PORTION OF THE AIR WITH THE LIQUEFIED PORTION TOPROVIDE AN IMPURITY-FREE GASEOUS FRACTION AND AN IMPURITY-CONTAININGLIQUID FRACTION; RECTIFYING SAID IMPURITY-FREE GASEOUS FRACTION TO FORMAN OXYGEN PRODUCT OF DESIRED PURITY AND A COLD GASEOUS NITROGEN PRODUCT;WITHDRAWISNG FROM A ZONE OF THE MAIN RECTIFICATION WHERE THE ARGONCONTENT IS HIGH, A VAPOR COMPRISING MAINLY OXYGEN AND ARGON; SUBJECTINGSUCH VAPOR TO AN AUXILIARY RECTIFICATION BY WASHING WITH A REFLUX LIQUIDFORMED BY PARTIAL LIQUEFACTION OF VAPOR FROM SAID AUXILIARYRECTIFICATION; PRODUCING SAID REFLUX BY A HEAT EXCHANGE BETWEEN VAPOR OFSAID AUXILIARY RECTIFICATION AND SAID IMPURITY-CONTAINING LIQUIDFRACTION TO SUBSTANTIALLY VAPORIZE SAID FRACTION AND FORM ANIMPURITY-CONTAINING CONCENTRATE; REMOVING SAID CONCENTRATE; PASSING THEIMPURITYFREE VAPORS FROM SAID VAPORIZATION TO THE MAIN RECTIFICATION;WITHDRAWING AN ARGON CONCENTRATE FROM SAID AUXILIARY RECTIFICATION; ANDUTILIZING AT LEAST THE NITROGEN PRODUCT FOR COUNTERCURRENTLY COOLINGINCOMING AIR.